Tubular and semi-tubular rivets are used extensively for fastening work-pieces which have pre-formed holes. Such workpieces can be fastened by inserting into the pre-formed holes a rivet which is longer than the combined thickness of the workpieces and then deforming outwardly (rolling) the protruding tubular end of the rivet to increase its diameter so it cannot return through the pre-formed holes in the workpieces.
The rolling action can be generated by the well-known process of positioning the rivet between a plunger and a setting die in a rivet setting machine and applying sufficient plunger force to cause the tubular end of the rivet to deform outwardly beyond its elastic limit in an approximately circular form. When the plunger force is removed the inherent elasticity of the tube material causes a certain amount of spring-back, but the residual deformation, or permanent set, ensures the rivet cannot return through the pre-formed holes.
Conventional rivet practice is to use the minimum length of tube to achieve the fastening required. Such an arrangement is shown in FIG. 1 in which a semi-tubular rivet 1 is inserted through holes provided in workpieces 3 and 5. If a simple splay set is adequate the tube 7 of the rivet can be very short, as shown in FIG. 2 of the accompanying drawings. For a more secure fastening it is normal to provide a tube length which allows the tube to roll outwardly through an angle .theta. of almost 180.degree., as shown in FIG. 3 of the accompanying drawings. Good practice, as illustrated in FIG. 3 of the accompanying drawings, requires that the angle .theta. of the roll of the tube is always less than 180.degree. so that, if the tube is deformed further after the leading edge has reached the workpiece, the leading edge will slide outwardly along the surface of the workpiece 5, as shown in FIG. 4 of the accompanying drawings. If the angle .theta. of the roll of the tube is close to 180.degree. the roll will be crushed against the workpiece whenever there is an over-stroking of the rivet setting plunger or when the workpiece is at or above its maximum expected thickness. With a roll angle .theta. of less than 180.degree. the contact between the rivet and the workpiece is always provided by the material which is on the outer surface of the original tube.